1. Field of the Invention
The present invention relates to ceramic compositions and methods for forming ceramic compositions. More particularly, the present invention relates to a method for making near-net-shape parts of refractory metal carbides.
2. Brief Description of the Prior Art
Reinforced silicon-carbide matrix composites having high-temperature applications requiring thermal and environmental stability and good thermal shock resistance are commonly used for combustion and exhaust components in jet and rocket engines, ceramic burner inserts, heat exchanger tubes, and the like. One example of fabricating such composites consists of the near-net-shape fabrication of silicon carbide parts using molten silicon infiltration of a porous carbon or fiber preform created by polymer pyrolysis. This method is typically referred to as xe2x80x9creaction forming.xe2x80x9d
An example of reaction forming is disclosed in U.S. Pat. No. 5,865,922 to Behrendt et al., in which a porous solid polymer is formed by reaction forming an infiltrated preform which is then pyrolized. According to the Behrendt et al. method, the resulting microporous carbon in the composite matrix is converted into silicon carbide.
Another example of reaction forming is disclosed in U.S. Pat. No. 5,945,166 to Singh et al. This patent discloses forming a ceramic composite fabricated from a refractory fiber preform, which is infiltrated with a polymer/resin mixture; cured at a temperature from 60xc2x0 C. to 90xc2x0 C.; heated to 600xc2x0 C. to about 700xc2x0 C. in an inert environment to convert the mixture to carbon; treated on one side with a paint or slurry containing germanium powder and a fugitive binder; treated on the other side with a boron powder containing paint or slurry; and both sides infiltrated with a molten silicon-metal alloy.
U.S. Pat. No. 4,940,679 to Claar et al. discloses self-supporting bodies produced by reactive infiltration of a parent metal into boron carbide typically resulting in a composite comprising a boron-containing compound and metal. U.S. Pat. No. 6,051,096 to Nagle et al. discloses a method of carbonizing cellulose-containing plants, which may be used to form ceramic-metal or ceramic-ceramic composites.
U.S. Pat. No. 6,013,226 to Steel et al. discloses metal carbide-containing refractory materials prepared by pyrolysing blanks comprising reactive metal sources and carbon-containing precursors under fluid pressure. Refractory composites containing ceramic fillers, reinforcing materials, such as carbon fillers, excess carbon or excess metal, are also disclosed by Steel et al. The method is used in the production of a range of metal carbide monoliths and composites with refractory properties.
A particular problem with the aforementioned refractory materials is the difficulty in forming complex shapes of the end product composites.
Fabrication of carbon preforms for molten silicon infiltration have been studied and optimized. Small amounts of refractory metals, such as molybdenum and niobium, have been added to the molten silicon to reduce residual silicon after infiltration by forming refractory metal silicides. The refractory metal silicides have the advantage of much higher melting points than pure silicon. Reaction forming, however, is limited to a single material, such as silicon carbide with only minor amounts of refractory metal silicides. Many other refractory metal carbides are of interest for their high-temperature properties such as zirconium carbide, titanium carbide, hafnium carbide, vanadium carbide, molybdenum carbide, niobium carbide, tantalum carbide, chromium carbide, and tungsten carbide.
Other methods for fabricating refractory metal carbide parts include machining, arc melting, and chemical vapor deposition. Because refractory metal carbides are hard and brittle, a net shape fabrication technique has great advantage over machining. The high melting temperatures of the refractory metal carbides make other net shape approaches, such as casting, difficult or impossible.
For example, one method for synthesizing very limited, rough shapes of zirconium carbide is arc melting in which a very high electrical current is passed through a mixture of zirconium and carbon to produce resistive heating, which melts the constituents to form zirconium carbide. Chemical Vapor Deposition (CVD) is another technique for producing refractory metal carbides in which a gas mixture is brought into contact with a high-temperature substrate. Chemical reactions at the gas-substrate interface produce the refractory metal carbide and can form complex shapes. CVD, however, is usually a very slow and expensive process requiring hours to produce micrometers of material.
There remains a need for a method to inexpensively provide monolithic, near-net-shape refractory metal carbides that can easily be configured into complex shapes.
The present invention relates to a reaction-forming method for producing monolithic, near-net-shape refractory metal carbides. The present method includes first fabricating an initial glassy carbon preform by casting an organic, resin-based mixture into a mold and subsequently heat treating the mixture in two steps.
The heat treatment consists of a low-temperature step carried out at a suitable temperature to cure the resin and a high-temperature step carried out at a suitable temperature to pyrolize the cured resin mixture leaving only a porous carbon preform. The amounts of reactants in the initial mixture can be varied permitting control over the density and microstructure of the carbon preform, which subsequently influences the microstructure and properties of the final refractory metal carbide material produced.
The glassy carbon preform is placed on a bed of refractory metal or refractory metal-silicon alloy pieces and heated above the melting point of the metal or alloy. The molten metal wicks inside the porous carbon preform and reacts, forming a refractory metal carbide or a refractory metal carbide plus a refractory metal silicide, depending on the composition of the infiltrating metal.
The refractory metal carbides of the present invention can be closely engineered by varying the constituents of the organic, resin-based mixture. The mixture may be advantageously cast into a wide variety of complex shapes, which are reflected in the shape of the refractory metal carbide produced by the present method.